Sunday, December 4, 2016

Lowland Snow: It's All About Temperature

Forecasting snow over western Washington is made hugely more difficult because our temperatures are often marginal for snow, with a few degrees making all the difference.

And the snow forecast for Monday is no different.  

Whenever air is coming off the relatively mild (roughly 50F) Pacific, it is difficult to get lowland snow unless the air is very, very cold aloft.

As scheduled, a strong cold front moved through this morning with cool onshore flow in its wake.  This transition was clearly shown in the winds and temperatures over Sea-Tac Airport (see time-height cross section below, red temperatures, blue are winds, time goes from right to left in GMT/Z/UTC, and heights are in pressure --850 is roughly 5000 ft).  The wind shifted around 0412 (Dec 4 and 12 UTC-4 AM) and the temperatures dropped.
It cooled enough aloft that a few areas above 500 ft or in intense showers (which brings the snow level down momentarily) that snow reached the surface at some Puget Sound locations.  Here is an example from Snoqualmie Ridge

Picture taken by Rob Nelson

Or Ken's Korner on Whidbey Island:
Picture courtesy of Susan Kieffer

The current snow threat is associated with cold unstable air that is being lifted by an upper level trough (or disturbance).  The latest infrared satellite image Sunday night show the convective showers associated  with the unstable air (the white mottled look).  An area of enhanced showers is off our coast and headed our way overnight--that is the immediate threat.

But although the air has cooled considerably, it is still marginal for snow near sea level and near the water.  In fact, the freezing level rose a bit during the day and is now (7:30 PM) roughly 2200 ft.  Keep in mind that the snow level (the level below which the precipitation is all rain) is about 1000 ft below the freezing level (the altitude of 0C).  Thus, the snow level is now around 1200 ft.
The approaching precipitation will bring the freezing and snow levels down (due to evaporation and melting of precipitation), but it will be difficult to bring snow all the way down to sea level, particularly locations near relatively warm Puget Sound.   Let's look at the latest model runs.

The NOAA/NWS HRRR (High Resolution Rapid Refresh) snow total through noon shows only .1-1 inch over most of Seattle, but much higher amounts south of Olympia and the Olympics.

The even higher resolution UW model run  for the 24-h snow ending 4 PM Monday is similar, but with more snow over Snohomish County and north Seattle (few inches).

The marginal (and thus uncertain) nature of this event is highlighted by the latest National Weather Service SREF ensembles for snow at Sea Tac (452 ft).  The mean total snow is around 3/4 of an inch, but there is a lot of variability.

The bottom line is that if you are near Puget Sound and close to sea level, you will see mainly rain showers, perhaps accompanied by less than an inch of wet snow.    But away from the water and a few hundred feet up, an inch or two is possible.  The roads should be ok other than steep hills since the soil is warm.  Higher up (400-800 ft), several inches are quite possible.  The main area of showers/snow should get to us roughly 5-7AM--so if you are starting your commute on a hill or inland, leave extra time.

The Seattle media has already started its favorite past-time, with "live storm coverage" and it is starting to have an impact on local supermarkets (see below).

And if you think the snow will be over tomorrow...think again.  Much colder arctic air will surge southward Tuesday morning, with potential snow in Sequim/Port Angeles and even northern Puget Sound.

New Weather Smartphone App, uWx

At the UW, we have developed a wonderful FREE weather app for Android smartphones that also collects pressure for use in weather forecasting. If you want to try it, please go to the Google PlayStore and download it.

Saturday, December 3, 2016

Snow Update with a Probabilistic View

I will have an update at 9 PM Sunday.... snow has already hit some higher-elevation locations and expect more tonight...

Let me update you on the snow situation, using all the technology at my disposal, both using ensembles for a probabilistic viewpoint and high-resolution forecasts.

Let's begin by noting an important point:  after the warmest November in Seattle history and mild temperatures the last few days, the road surfaces are quite warm, which will help melt any light snow falling on them. To illustrate, here is the latest air and road temperatures (road surface temperatures in rectangles) from the Seattle SnowWatch website.  Most roads are in the mid to upper 40s.   This will be a powerful weapon against icing during any initial onslaught of snow.

As in any good forecast we should start from a probabilistic viewpoint with a clear understanding of forecast uncertainties, and as I have discussed many times in the past, ensemble forecasts (using many forecasts, each a little different) is a powerful tool to do so.

The National Weather Service 21-member GFS ensemble's cumulative snow forecast for Seattle (below) has all members (forecasts) going for some snow this week (remember this is amount of snow falling out of the sky, not depth on the ground).   Black is the average of the all the forecasts and blue is the operational high-resolution GUS run. As noted earlier, there are two possible events.  The first, on Monday, produces about 1-2 inches and the late Wednesday/early Thursday event has a little more (but more uncertain).
The more skillful European Center ensemble system, with more ensemble members, higher resolution, and better physics is also showing snow in two steps (see below).  About a quarter of its members suggest no snow, while the rest produce generally light amount (.5-1.5 inches).  The second event is much larger with 3-6 inches for several members.  

The National Weather Service also has a short-range ensemble system using two models (NMM and WRF) and higher resolution (about 16 km).   This system shows a mixture of solutions for the first potential snowstorm, with a lot of uncertainty and about a quarter of runs showing very little snow.

So what is my take away from all this?    There is uncertainty, with about 25% of the forecasts showing virtually no snow.  Roughly 75% of the model runs are predicting an inch or two at Seattle-Tacoma Airport (452 ft).    The temperatures on Monday are colder than we have had, but they are still on the margin near sea level and the water.   So expect a substantial gradient with elevation (more on the hills).  

To get a better spatial view of the snowfall, lets look at the UW WRF, which is driven by the NWS GFS operational model.   Since the GFS solution is close to the mean of the ensemble and seems to have a representative solution, the UW ultra-high resolution view should be useful.

Here are the 24h snow totals from the UW 1.3 km domain (very high resolution in the weather business).  First the 24h ending 4 AM Monday.  Scattered snow showers, with heaviest lowland values south and southeast of the Olympics, which is not unusual with relatively warm snow events and southerly/southeasterly flow.  Not much in Seattle--thus the Seahawks game should be fine.  Some snow over SW Washington.

But the next 24 h, ending 4AM Tuesday is another story. The Puget Sound area gets a half to a few inches, with north Seattle to Everett getting more than the south Sound.  Less near the water.

 The 9-km European Center snow total through Monday evening shows loads of snow over the mountains (12-15 inches) and zero to a few inches over the lowlands (with much less near the water).

The bottom line of all this?  Nothing is certain.  But it appears probable (say at the 75% level) that much of the lowlands away from the water will see some light snow on Monday, particularly above a few hundred feet and over north Seattle and southwest of the Olympics.  

We will leave the Wednesday event for another blog.

Thursday, December 1, 2016

Significant Cold/Snow Event Next Week

Will update about snow at 2 PM Saturday

As we approach the weekend, it is becoming increasingly clear that a major cold/snow event will occur next week, but with substantial uncertainty about lowland snow.  One thing is certain: it will be an extraordinary week for snow in the mountains, and everyone who enjoys mountain snow recreation should get ready for a wonderful period of light, bountiful snow.

Right now the snowpack is still a bit below normal over about half of the region, with near normal snowpack (green colors below) and crazy above-normal snowpack in a few areas (the Olympics are over 250% of normal!)  By the end of next week the entire mountain region should be above normal.

Early next week will be the coldest period since last January, with temperatures dropping into the 20s over western Washington..... so drain you hoses and protect vulnerable plants.

The models, both high-resolution and ensembles, are in agreement that we will have a much colder period early next week.  The NWS GEFS ensemble system (21 members in gray, ensemble average black, GFS high-resolution member-blue), show unanimity of solution towards colder temperatures in Seattle early next week (highs in mid-30s, lows in the mid-20s), with modest warming at the end of the week (see plot below)
The European Center ensemble has the same idea.

During the next week, the cold and snow will occur in roughly three acts.  The first act, as shown by the low-level temperatures (colors), winds, and sea level pressure at 4 AM Sunday,  will encompass a strong cold front that will move southward across our region in Sunday, bringing much colder air (see below).  By later on Sunday, the air above our region will be cold enough for snow to reach near sea level.

In the second act, which will take place on Monday, cold air will extend over Washington and Oregon with frigid conditions to the east of the crest of the Rockies.  A low center will form over western Washington (map at 4 PM Monday shown below).  How the low develops and positions itself will determine how much snow falls in the lowlands.

In the third, and perhaps the most dramatic act, a weak occluded system approaches the coast bringing warming temperatures aloft and precipitation.  With cold air in place over western Washington, the precipitation will start as snow at low levels, and perhaps a lot of snow.

Now lets look at the ensemble snow forecasts.  The NOAA/NWS GEFS ensemble show a lot of variability (and thus uncertainty) with the forecast, with the average of all the forecasts (black line) and the high-resolution forecast (blue line) looking very similar.  There are a few inches on Monday, a gap, and then a lot more snow later Wednesday and Thursday.  All the models show at least 3 inches by the end of the period.

The European Center ensemble shows a similar picture:  light snow Sunday/Monday, but heavier amounts Wednesday/Thursday.

Now, to see the spatial distribution better, lets turn to ONE forecast, the UW high-resolution prediction based on the NOAA/NWS global model.  For the 24-h period ending 4 PM Monday (below), there is plenty of snow over the region, some of which extends to ground level over Puget Sound an SW Washington.
But the 24h snowfall ending 4 AM Thursday really is impressive...a foot over the mountains and several inches over the lowlands.

So at this point, much colder temperatures is virtually guaranteed Monday through Wednesday, and there is consensus among the ensembles that we will get at least some light snow at low levels.  The warm road temperatures will help to mitigate the impacts on highways in contact with the soil.

Tuesday, November 29, 2016

Will Snow Hit the Puget Sound Lowlands in a Few Days?

Some of the media have already started to talk about it: the possibility for much colder temperatures and lowland snow during the Sunday through Tuesday period.

Let's analyze the possibilities, making use of the most powerful probabilistic forecasting tools at our disposal.  We will attempt to avoid the problems experienced during the October 15th storm by highlighting the forecast uncertainties and the use of ensembles.

Snow Forecasting is HARD

Let me begin by noting a key point:  it is difficult to get lowland snow around Puget Sound because the Pacific Ocean and Puget Sound are relatively warm (around 50F).  It is easy for us to be mild and wet and frequently we are cool and dry.  But to be cool and wet is very hard, demanding a rare configuration of pressure and winds.  As a result,  snow is the most difficult forecast problem in our area.

As I have mentioned frequently, all forecasts should be probabilistic and the most potent tool to create probabilities and to determine uncertainty is through the use of ensemble forecasts--running weather models many times with different initial states and model physics (e.g., moist processes).

So let's look at some ensembles!  We can start with the NOAA/NWS GEFs ensemble of 21 members (relatively coarse 35 km grid spacing) for snow over the next week (see below).  Dates/times are in UTC/Z, the average of all the ensemble members (the ensemble mean) is shown in dark black, the individual ensemble members are in light gray, and the blue line is single high-resolution (13 km)  GFS forecast.

Note that the ensemble mean shows about 1.5 inches by 4 AM Monday (12Z, 5 December) and there is considerable spread (from 6 inches to 0).   Most members produce 1-4 inches.  The high-resolution run is very different, with no snow until later in the week.

Next, lets examine the snow forecast of the best global ensemble system in the world (the European Center) using the wonderful WeatherBell web site. The European Center (EC) ensemble is larger (51 members) and higher resolution (25 km) than the US version.  The graphic below will take some getting used to.

The top panel shows the accumulated snowfall for each ensemble member for Seattle. Note that most members show snow starting on Sunday or Monday (Dec 4th or 5th) and several members show much more snow later in the week (8th or 9th).

The bottom panel shows the ensemble mean (green) and the high resolution snow forecast (deterministic, blue) for Seattle.   The ensemble average shows some very light snow on Sunday and Monday (maybe a half inch), but more more later in the week (with total accumulation of a few inches).  The much higher resolution deterministic run (single 9-km grid spacing run) shows more snow (3-4 inches).

What about temperature in Seattle?  Here are the ensemble forecasts for high and low temperatures from the EC model.  The single high resolution forecast is black, the ensemble mean is green, and the range of the forecasts are shown by the blue "whiskers".   The rectangles encompass the 50% of forecasts closest to the mean.

The big story....a major cool down, with daily highs dropping from 46-47F to around 40 F on Monday.  Even lower after that.  After around 200 h, the uncertainty becomes very, very large.

So based on the ensembles, I feel pretty confident to forecast much cooler temperatures next week.  And the ensembles suggest a good chance of getting at least a dusting early next week, with more later.

Finally, with all that I have said about uncertainty, lets look the latest UW high-resolution run, which is driven by the high-resolution NOAA/NWS GFS model.

On Sunday at 1 AM, a very strong Pacific cold front is pushing southward over NW Oregon, with cold temperatures behind.

By 4 AM Monday, cold air (blue/purple/white colors) had spread over our region.

By 1 PM Monday, a low-center had formed just off of the SW WA coast-- this is getting close to a typical snow pattern for Puget Sound.

The 24h total now forecasts ending 4 AM Sunday indicates LOTS of snow over the WA Cascades (skier will rejoice)
The subsequent 24 h brings lowland snow, mainly over SW WA and western Oregon.  But the uncertainty is very large, so be prepared for the details to change.

Bottom line of the above:  there is a high probability that we will see substantially colder temperatures over the region starting Sunday.   It is probable but not certain that there will be some light snow at lower elevation on Monday (with the potential for more in some locations).   There is a good chance of heavier snow (a few inches) later in the week.  At this point, the ensemble don't indicate a major regional low level snow.    The mountains will get large amounts of snow, including the lower passes.

Monday, November 28, 2016

Why Does Mount Baker Get So Much Snow?

Mount Baker Ski area, located in the northern Washington Cascades (see map below), is a fabled

location for large amounts of snow.  For example, it holds the U.S. record for annual snowfall (1140 inches).

And today, the Mount Baker Ski Area website is claiming that they have more snow than any other ski facility in the entire U.S.!  With 83 inches at 5000 ft!

Are these Trumparian claims?  Is Mount Baker really that snow bound?  And if so, why are they so endowed with the white stuff?

A glance at the snow depth analysis from the NOAA National Hydrological Center shows that the greatest snow depths (darker blues and purples)  today are clearly over our part of the country,
A closer view shows heaviest snow around the volcanic peaks, the north Cascades and the Olympics.
The Hurricane Ridge ski area in the Olympics is not open yet, so I think that the Mount Baker ski area has reason to crow that they are tops in the U.S. right now.  And the all-time record was pronounced by an official arm of the U.S. government, so I am certainly not in a position to dispute their findings.

So what is it about Mount Baker ski area that gives it so much snow?  It is really not that high, with most of the ski runs between 4000 and 5000 ft.   Folks in Colorado or the Sierra would laugh at such low elevations.

But Mount Baker and much of the Cascades start with a big advantage:  lots of moisture and precipitation.  The western slopes and crests of the Northwest mountains are the wettest locations in the U.S. due to the persistent winter storms passing over the region in winter and the great enhancement by the regional terrain (see annual climatological precipitation maps below).

The north Cascades are not as wet as the western side of the Olympics, but are far enough to the northeast of the Olympics  that they escape much of the Olympic rain shadow. Furthermore, the terrain around Mount Baker extends more westward than the bulk of the nearby Cascades and has more of a NW-SE orientation, providing more uplift to the frequent southerly/southwesterly flow of the region.

Thus, with many incoming storms and moist air off the Pacific moving up the terrain (and thus cooling, saturating, and then precipitating), moisture is no problem.   The best supply in the U.S.

And there is not much high terrain upstream of Baker for the typical southwesterly winter flow;  there there is less competition for the incoming moisture.

But then there is the issue of temperature...the air must be cold enough to snow. That average freezing level (or melting level) of the air coming into our region (as observed at the Quillayute radiosonde site) is shown by the graphic below (from the wonderful NOAA/NWS Storm Prediction Center website).  On average, the freezing level is near 4000 ft.  And keep in mind that snow does not melt immediately at freezing...the snow level (where all the snow is melted) is about 1000 ft below the freezing (or melting) level.

So Baker is just high enough to get reliable snow.  And, of course, being in the northern part of Cascades makes it cooler than, say, central Oregon.

 But there is something else.   The amount of snow is enhanced for temperatures just below freezing, because the amount of moisture that air can hold (and thus precipitate) is greatest for warmest temperatures.   You can get heavier snowfalls in general for temperatures between 27 and 32F then 15 and 20F.    So Baker is at the perfect altitude range for maximum snow.  To put it another way, a ski are at 4000-5000 ft gets more snow than at 6000-7000 ft with an average 4000 ft freezing level.

Lots of moisture, great exposure to incoming flow, orographic/upslope precipitation enhancement, and perfect elevation range for the observed freezing level come to together for an optimum environment for lots of snow.

Finally, let's end this blog by looking at the snowfall and accumulation at  the Baker observing site maintained by the Northwest Avalanche Center.  The top figure shows 24h snowfall and the bottom,  snow depth.  About 65 inches at Baker, with the big dumps on Nov 24th and 25th.

More is coming.

Saturday, November 26, 2016

Wet and Warm Thanksgiving Weekend After Large Amounts of High-Elevation Snow

The Thanksgiving has been generally wet and warm, but with large amounts of snow at higher elevations.  The temperatures at Seattle-Tacoma Airport and Stampede Pass (4000 ft) in the Cascades, tells the story (red and blue lines are average highs and lows this time of the year).   Minimum temperatures have been way above normal at both locations.  As I have noted in several blogs, this warmth is due to a persistent area of low pressure off our coast, which has brought in warmth and moisture.

The precipitation over the past day (24h ending 11 AM Saturday) has been impressive over the southern Olympics,  and north Cascades (2-3 inches in places).  You will note a profound rain shadow NE of the Olympics with only .07 inches over northern Whidbey Island (Ebey's Landing is one of my favorite winter walks).

The past seven days has been VERY wet along the coast, with some locations along the slopes of the Olympics and coast coastal mountains getting 10-20 inches)---see left panel below.  The right panel shows the difference from normal for the same period--unusually wet coast.  Some of the coastal river have hit or approached flood stage.

On Thursday and Friday, it was cool enough (and certainly wet enough) that the higher Cascades (above roughly 4000-4500 ft) got hit with several feet of snow (like three feet at Mount Baker ski area).  But it was too warm for the lower ski areas (and particularly Snoqualmie) to get anything on their lower slopes.

And it was made worse last night, when a warm front came through, pushing the snow level up to around 5000 ft.  The time-height cross section above Seattle shows this clearly (red is temperature in C, blue wind barbs, ordinate is pressure (850 is about 5000 ft), and time increases to the left (in UTC)

A weak frontal zone is now draped over the Northwest coast with light to moderate precipitation  (see radar below)
As the day goes on the front will slip eastward with precipitation stopping and the air cooling aloft.

Sunday morning will be a good time for outdoor activities (or to take part in the Seattle marathon) or to get across the passes.   Later Sunday, a strong (but cooler) frontal system will move in and will it should be raining over the lowland by dinner time.